Method of making printing masks with high energy beams

ABSTRACT

A STENCIL MASK SUITABLE FOR PRINTING ELECTRONIC CIRCUITS AND THE LIKE IS MADE BY APPLYING A BEAM OF HIGH-ENERGY EMMISIONS SUCH AS AN ELECTRON BEAM OR A LASER BEAM TO AN ORIGINAL BLANK COMPRISING TWO LAYERS OF DIFFERENT VOLATILLZABILITY IN RESPONSE TO IMPRINGMENT BY THE BEAM. THE BEAM IS APPLIED TO THE LAYER WHICH IS MORE READILY VOLATILIZED TO FORM A GROOVE THERETHROUGH BOTTOMING AT THE INNER SURFACE OF THE SECOND LAYER. PERFORATIONS EXTENDING ENTIRELY THROUGH THE BLANK ARE PRODUCED WHERE DESIRED BY INCRESING THE BEAM ENERGY APPLIED TO SELECTED POINTS ALONG THE BOTTOM OF THE GROOVE. BECAUSE THE SECOND LAYER IS LESS READILY VOLATILIZED, UNCONTROLLED VARIATIONS IN THE DEPTHS OF THE GROOVES AND THE DANGER OF HAVING A GROOVE BREAK ENTIRELY THROUGH THE BLANK WHERE THIS IS NOT DESIRED ARE MINIMIZED.

0. A. SHORT June 6, 1972 METHOD OF MAKING PRINTING MASKS WITH HIGHENERGY BEAMS 2 Sheets-Sheet 1 Filed June 10, 1970@oooooooooocoooooooooooooooooooov AOOOOOOOOO OOOOOOOOODOOOOOO 000000 0ADOOOOOOOOOOUOO COO 000000000 00 0O 0000aFoo0oooooooooooooooooooooooooooo000000008 mvzm'oa; O LIVER A. 5H0 RTATTORNEY June 6, 1972 Filed June 10, 1970 O- A. SHORT wi l:

zsheets-sheeyz mvcmoR: OLIVER A, SHORT AT TORNEY United States Patent O3,668,028 METHOD OF MAKING PRINTING MASKS WITH HIGH ENERGY BEAMS OliverA. Short, Wilmington, Del., assignor to E. I. du Pout de Nemours andCompany, Wilmington, Del. Filed June 10, 1970, Ser. No. 45,157 Int. Cl.B41c N14 US. Cl. l56-3 9 Claims ABSTRACT OF THE DISCLOSURE A stencilmask suitable for printing electronic circuits and the like is made byapplying a beam of high-energy emissions such as an electron beam or alaser beam to an original blank comprising two layers of differentvolatilizability in response to impingement by the beam. The beamBACKGROUND OF THE INVENTION This invention relates to improvements inmethods for making printing masks by controlled application of a beam ofhigh-energy emissions to a blank.

There are a variety of applications in which it is desirable to apply avery accurately delineated configuration of printing material to asuitable substrate. One particular application of such a process is inthe fabrication of socalled thick film printed electronic circuits. Inmaking such devices, typically a conductor ink is applied or printedselectively onto predetermined portions of an ap propriate ceramicsubstrate, and subsequently fired onto the substrate. In many cases,requirements of miniaturization or compactness make it desirable toprovide such printing with as high a degree of definition as possible,e.g. in the form of one-mil wide stripes separated by three mils or lessbetween the centers of adjacent stripes.

Techniques for attempting such printing are known which utilize screenstencils, made for example of a screen of fine-mesh wovenstainless-steel wire or nylon filaments, selectively coated with amixture of polyvinyl alcohol and .polyvinyl acetate to close the meshopenings in the areas where printing is not desired. However, theresolution and accuracy of printing heretofore obtained with suchmethods has been less than is desirable for some purposes, e.g. amaximum resolution of the order of S-mil *width lines separated by 10mils between their centers. Better resolution has been obtained usingmetal foil printing masks made by chemical etching or electro-formingtechniques. Typically the total thickness of such a mask is of the orderof one to two mils, and the pattern of the desired printed circuit isetched into one side of the mask, half-way through the thickness of thefoil, and a pattern of closelyspaced indentations is etched into themask from the other side to produce perforations through the mask at thebottoms of the grooves. While line resolutions of the order of 2-milline width and four-mil center-to-center line spacing have been obtainedin some instances, the accuracy and reproducibility obtainable with suchmasks has sometimes been less than desirable, principally because ofundercutting of the mask material during etching or excessivebuildup ofmaterial during electro-forming. A further drawback is that the lattermethod involves ph0tographic techniques using photoresists for definingthe the areas of the mask blank exposed to the etching and/ orelectroforming operations, and consequent complication and expense inpreparing the photographic master through which light is projected toform the pattern-defining image on the photoresist.

It is an object of the present invention to provide a new and usefulmethod for the fabrication of printing masks.

A further object is to provide such a method which is capable ofproviding grooves or other depressions in a printing mask blankreproducibly and reliably.

It is also an object to provide a method of making a printing mask blankreproducibly and reliably. energy emissions to a mask blank in which theaccuracy and reproducibility with which grooves or other depressions areformed in the blank are improved.

SUMMARY OF THE INVENTION These and other objects and features of theinvention are achieved by the provision of a method for making aprinting mask by application of a controlled beam of highenergyemissions to a mask blank, in accordance with which method the blank isprovided with at least two layers, one of which is more readilyvolatilized than the other in response to impingement by said beam, andthe beam is applied to the side of said one layer remote from said otherlayer to remove at least portions of said one layer Without perforatingsaid other layer, thereby to form a groove or other depression.Preferably the parameters of the beam are controlled so that thedepression formed extends to said other layer but does not penetrate itsubstantially. Additional beam energy may be applied to cut entirelythrough both layers where a perforation is desired. Using this method,the removal of the material of said one layer tends to be slowed orarrested when the second layer is exposed, and a reproducible depth ofgroove or other depression therefore more readily obtained with lesscriticality of adjustment of the beam parameters such as beam size, beamenergy, beam current and beam deflection rate.

BRIEF DESCRIPTION OF FIGURES These and other objects and features of theinvention will be more readily understood from a consideration of thefollowing detailed description, taken in connection with the accompanydrawings, in which:

FIG. 1 is a plan view illustrating schematically a form ofprinted-circuit module which may be made by the method of the invention;

'FIG. 2 is a sectional view taken along lines 22 of FIG. 1;

FIG. 3 is an elevational sectional view illustrating how such a printedcircuit may be formed by means of a stencil mask;

FIG. 4 is a plan view of a stencil mask suitable for use in printing theprinted circuit of FIG. 1;

.FIG. 5 is a fragmentary plan view of a portion of the mask of FIG. 4;

FIG. 6 is a sectional view taken along lines 66 of FIG. 5;

FIG. 7 is a sectional view taken along lines 77 of FIG. 5;

FIG. 8 is a fragmentary perspective view of a mask blank from which astencil mask is to be made;

FIG. 9 is a schematic representation, partly in block form, illustratingan arrangement of apparatus suitable for practicing the method of theinvention in one of its forms; and

FIGS. 10 and 11 are perspective views showing a portion of a printingmask in various stages of fabrication in accordance with the invention.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS Referring to the drawingsby way of example only, FIGS. 1 and 2 show a printed-circuit modulecomprising a ceramic printed-circuit substrate having adherent to itsupper surface a pattern of conductors such as 12 of a conductive ink,e.g. a substance which has been applied to the substrate in conformancewith a predetermined pattern and thereafter fired to provide suitableelectrically-conductive paths on the substrate. Such printed circuitmodules and their uses and applications are well known to those skilledin the art of printed circuitry and need not be described furtherherein.

FIG. 3 illustrates one method of forming such a printed-circuit patternby placing a stencil mask 14 over the top surface of the substrate 10and forcing the conductor ink 1'6, which may be in a thin paste, throughopenings in the stencil onto underlying parts of the substrate byoperation of the squeegee 18. The mask 14 has been suitably grooved onthe surface adjacent the substrate 10 on the opposite face so that theconductor ink is forced through the apertures and along the grooves intocontact with the surface of the substrate 10 where printing is desired.The mask is then removed and the sub strate and ink fired in a furnaceto form a permanent pattern of conductive paths, as desired. Thisprocess, and configurations of the mask suitable for effecting thevarious desired printing patterns of, are well known in the art and itis an object of the present invention to fabricate such a mask with ahigh degree of reliability, accuracy and resolution, and withoutrequiring photographic steps.

More particularly, FIGS. 47 show in greater detail a form of mask 14which can readily be made by the method of the invention. In thisexample the mask 14 may comprise a plurality of grooves such as 20, eachextending only partially through the thickness of the mask, and aplurality of spaced-apart apertures such as 22 extending from the bottomof each depression entirely through the remainder of the thickness ofthe mask to the opposite surface thereof. The grooves such as 20 defineregions throughout which ink is applied to the printed-circuitsubstrate, and the apertures 22 provide the openings through which inkis supplied to these grooves, as discussed above in particularconnection with FIG. 3.

As is shown particularly clearly in FIGS. 6 and 7, in accordance withthe invention the mask 14 comprises a pair of bonded layers or laminas26 and 28, the grooves such as 20 extending through the upper lamina 26and the apertures or perforations extending through the lower lamina 28.The upper lamina 26 is substantially more readily volatilized inresponse to impingement by a beam of high-energy emissions than is thelower lamina 28. There are a variety of materials suitable for use asthese lamina, and for the purpose of the present example it will beassumed that lamina 28 is a nickel foil about 1 mil in thickness andthat lamina 26 is a layer of cadmium of about the same thicknesselectro-plated onto the upper surface of the nickel. The reasons for,and the advantages of, this laminated construction of the mask will beset forth more fully hereinafter.

FIG. 8 shows a portion of the plane laminated mask blank 14 which isused as the starting blank in making the mask stencil, and FIG. 9illustrates schematically one arrangement of apparatus by which apattern of grooves and holes suitable for an electronic-circuit printingmask may be provided in this mask blank, rapidly and automatically.

Referring to FIG. 9, a machine bed 30' supports a worktable 32 which iscontrollably positionable with respect to the bed 30 along two mutuallyorthogonal directions in a horizontal plane. Such motions may beprovided by worktable control motor 35 geared to drive the worktablealong one direction with respect to a frame. 36,. and a motor 37 gearedto drive frame 36 along the orthogona direction with respect to themachine bed 30 On the upper surface of worktable 32 a support block 40is appropriately fixed in position with respect to the worktable andcarries the laminated mask blank 14 on its upper surface. Appropriateclamping arrangements such as 44 may be utilized to clamp the laminatedmask material releasably in position on top of the support block 40.

The upper surface of the mask 14 is impinged by a fine focused beam 50of high-energy electrons from the controllable high-energy electron beamsource 52. Electron beam source 52 may be of a type known in the art andcommercially available, and which has been used in the prior art formetal working purposes.

Electron beam source52 includes four control terminals-a focus controlterminal 56, a beam-energy control terminal 58, a beam-deflectioncontrol terminal 60' and a beam-current control terminal 61, responsiveto electrical signals to control, respectively, the focus, energy,angular position and current of the electron beam 50. A programmedcontrol computer 64 is provided with five output control terminals 66,68, 70, 71 and 72 connected by electrical .lines 74, 76, 78, and 81respectively to terminals 56, 58, 60 and 61 of electron beam source 52and to a table-position control terminal 84 of the worktable control box85 from which operation of motors 35 and 37 is controlled by connectionsnot shown. Preferably, the mask blank, worktable system and control boxare enclosed in a chamber in which a partial vacuum is maintained bymeans of a low-vacuum exhaust system 92, such as is employed inelectron-beam milling of large objects.

Control computer 64 is appropriately programmed by conventional means toprovide, at its five output control terminals, appropriate coordinatedcontrol signals for controlling the focus, energy, current and positionof electron beam 50 and for controlling the position of the mask 14 withrespect to the electron beam source 52. The program incorporated in thecontrol computer 64 may be of any desired form for varying the point ofimpingement of the electron beam 50 upon the upper surface of thelaminated foil 14, as well as for varying the focus, energy and positionof the beam all according to a predetermined routine. It is noted thatthe relative position of the beam 50 with respect to the foil 14 may bealtered either by motion of the worktable or by electrical control ofbeam position, and in general grosser and slower displacements of therelative position of the beam will be accomplished by motion of theworktable while the finer motions required for the rapid tracing-out ofdesired grooved patterns and for the formation of apertures willnormally be provided by electrical deflection of the electron beam.

In general, the rate at which material of the mask is removed dependsupon the number and energy of electrons striking a given region of themask per second and upon the evaporation or volatilizationcharacteristics of the material impinged; the total amount of materialremoved also depends upon the time for which the beam impinges a givenregion. Accordingly, more material will be removed from the mask, ingeneral, when the beam dwells in one position for a substantial periodof time or moves slowly with respect to the mask; when the energy of theelectrons is increased by increasing the accelerating voltage to whichthey are subjected; when the beam current is increased; and when thebeam is more sharply focused. By control of these parameters grooves areformed extending only partially through the layer 26 and holes entirelythrough the material at the bottoms of the grooves are produced byincreasing the beam energy, beam current, or beam concentration or byapplying the beam for a longer time at a given position where the holeis to be made.

More particularly, as represented in FIG. 10, the beam 50 is deflectedalong the paths on the surface of the upper lamina 26 in which grooves20 are to be formed with a deflection speed, beam energy and beamconcentration sufficient to assure cutting of the grooves down to theupper face of the lower lamina 28. When the beam is traversing a regioninwhieh'no groove is to be formed, it is possible for the computerto'make the beam energy I zero by reducing the accelerating voltage,although alternatively, and generally more easily, the beam current may"be cut olf by voltage applied to the eiectron gun from the computer,for example. The-material of the lower "lamina 28, being much lessreadily volatilizable than the top mask layer 26, will not be removed atany appreciable the grooves again, to. dwell at spaced-apart pointsalong the groove, to assume an increased beam energy while at thesepoints,-.and to be substantially blanked out or at least maintained. atalower energy level while travelling between these points. The higherenergy beam, while "dwelling at the successive spaced-apart points,produces Lholes through the lower lamina 28 at these points, asillustrated in FIG. 11,thereby providing the desired mask configurationconsisting of grooves in the upper surface extending only partly throughthe mask and holes at spaced intervals along the grooves extendingthrough the *remainder of the mask for supplying ink to the groovesduring the printing process. I

In another form of themethod, the electron beam may be controlled soasto produce the grooves and the perforations during the same scanningof the mask, by I providing a speed of beam deflection and a beam energysufficient to remove only the toplamina of materialwherc perforationsare not required,.and by providing a lower deflection speed and/ or ahigher beam energy while impinging points at which holes are to be'produced by pen'etration entirely through both layers. With thisprocedure it is made entirely certain that the holes and grooves will beaccurately aligned, since they are formed in the same scanning of a lineby the beam.

The thickness and nature of the materials used for the mask and theparameters of electron-beam energy, concentration, focus and rate ofmovement, as well as the pattern traversed by the beam may all be variedsubstantially indifferent applications. Examples of suitable mate rialsfor the upper (first impinged) lamina 26 include zinc, cadmium, indium,tin, bismuth, lead and synthetic plastics; ,suitable materials for thelower lamina 28 inelude nickel and its alloys, copper and its alloys,iron and its alloys, chromium, molybdenum, and tungsten. In a preferredform, the upper layer is cadmium, electroplated onto a lower layer ofnickel. In each case the upper layer is readily volatilized under beamconditions which have a relatively smaller effect in volatilizing thematerial of the lower layer, with the advantages set forth above.

In one specific example of a preferred procedure for forming a mask of atype suitable for elfecting printing of electronic circuits, a l-milthick foil of nickel is electroplated with a 0.8 mil thick layer ofcadmium to provide a laminate foil about 1 inch by 1 inch square, whichis then secured to the support block 40 on the worktable 32. Theelectron beam source 52, initially turned off, is turned onautomatically by the control computer 64 and deflected along the desiredpattern of grooves with a spot size of the beam on the mask surface ofabout 2 mils in diameter and with a beam energy just sufficient toassure complete removal of the upper-lamina material along the groove,the beam being automatically cut off by signals from the controlcomputer when the beam is moving between portions of the foil in whichno groove is to be formed. The resultant grooves extend entirely throughtheupper lamina 26 and have a width of approximately 2 mils, andadjacent grooves can be formed with their centers spaced-apart from eachother by about 4 mils. 'The' beam, with a spot size typically of about1.5 mils in diameter, is then automatically caused to traverse thegrooves again, dwelling at intervals spaced-apart along the beam path byabout 3 mils, with a beam energy at the dwell points sufiicient toproduce spaced holes extending entirely through the lower lamina 28 atthe bottoms of the grooves, as desired. With such a procedure, a patterncomprising grooves covering about half the area of the 1" by 1" squarecan be produced in two or three minutes. The-maskthus produced is thensuitable for use in conventional electronic circuit printing. Anotherform of beam of high-energy emanations which may be utilized in place ofelectron beam is a laser light beam of high intensity. Such a beam canbe controlled as to its point of impingement by optical means,

and its energy, beam width and focus can also be controlled by knownmeans. Accordingly, in another embodiment of the invention, grooves andperforations in a mask suitable for printing of electronic circuits aremade by traversing a high-intensity laser beam along the line regions inwhich grooves are to be produced on the upper surface of a laminatemask, the upper lamina of which is -more readily volatilized by thelaser than is the lower lamina, after which the laser beam is appliedfor substantial periods of time at higher intensities to spacedapartpoints on the bottom of the grooves to produce the desired holesextending through the lower lamina.

Suitable structures of apparatus for producing such a laser .beam and/orcontrolling its parameters and position to {effect such opertion will beapparent to one skilled in the art. Again, if desired, the formation ofthe grooves and of the holes may be accomplished on the same scan bysuitable controlof beam energy and speed.

While the invention has been described with .particular reference tospecified embodiments in the interest of complete definiteness, it willbe understood that it may be embodied in a variety of forms diverse fromthose specifically shown and described Without departing from the scopeof the invention as defined by the appended claims.

What is claimed is:

1. A method of fabricating a printing mask comprising the steps of (a)forming a mask blank, one layer of which is more readily volatilizedthan another layer adjacent thereto in response to impingement by a beamof highenergy emissions, (b) impinging such a beam of highenergyemissions on the side of said one layer remote from said other layer,and (c) traversing the point of impingement of said beam on said surfacealong a line segment on said side of said one layer while controllingthe energy of said beam within a predetermined range to producevolatilization and removal of said one layer along said line segmentwithout penetrating substantially into said other layer, thereby to forma groove in said one layer and (d) impinging said beam at spaced apartpoints along said line segment with an energy above that used in step(c) to perforate said other layer and thereby produce aperturesextending entirely through said mask blank at said points.

2. The method of fabricating a printing mask for printinfg electricalcircuitry on a substrate, comprising the steps 0 forming a laminate maskblank in the form of a foil comprising a first lamina of a firstmaterial adjacent and bonded to a second lamina of a second material,said first material being more readily volatilizable than said secondmaterial in response to impingement by a beam of electrons;

generating a beam of electrons and impinging said beam upon the surfaceof said first lamina on the opposite side thereof from said secondlayer;

traversing the point of impingement of said beam on said surface along apredetermined pattern of line second layer, thereby to form grooves insaid first layer; and

impinging said beam on said mask blank at spaced apart points along saidline segments with an energy above said range to produce apertures.extending entirely through said mask blank.

3. The method of claim 2 in which said forming of said mask blankcomprises electrodepositing oneof said first and second materials upon afoil of the other'of said first and second materials.

4. The method of claim 2 in which said'first material is selected fromthe group consisting of zinc; cadmium, indium, tin, bismuth, lead, andsynthetic plastics, and said second material is selected from the groupconsisting of nickel and its alloys, copper and its alloys, ironand itsalloys, chromium, molybdenum, and tungsten.

5. The method of claim 4, in which said first material is cadmium andsaid second material is nickel. i

6. The method of fabricating a printing mask for'printing electricalcircuitry on a substrate, comprising the steps of forming a laminatemask blank in the form of a foil comprising a first lamina of a firstmaterial adjacent and bonded to a second lamina of a second material,said first material being more readily volatilizable than said secondmaterial in response to-impingement by a laser beam;

generating a laser beam and impinging said beam upon the surface of saidfirst lamina on the opposite side thereof from said second layer;

traversing the point of impingement of said beam on said second layer,thereby to ay ran v.

. 1 ,impinging saidbeam on said,- mask blank at spaced apart pointsalong said line segments withan energy above said range to produceapertures extending en- H .-tirely throiighsaid mask blank. I I H 7. Themethod of claim 6,,vin which said forming ,of said mask blank compriseselectrodepositing one ofisaid first and second materials upon a-foil ofthe other of 'said first and second materials. 8. The method of claim 6in which said first material is selected from the group consisting ofzinc, cadmium, indium, tin bismuth, lead, and synthetic plastics; andsaid second material is selected from the group consisting "of nickeland'its alloys, "opper andits alloys, iro'r r afnd its form grooves insaid alloys, chromium, molybdenum, and tungsten; 9. The method of claim8, in which said first material is cadmium and said second materialisnickel.

I References Cited UNITED STATES PATENTS. 3,436,468 4/1969- Haberecht'-2 -117 47 X 3,260,625 '7/1966DellaPergolaetaha;117--8UX 3,293,65212/1966 Roshon 6123.1. ....117-68X 3,330,696 7/1967 Ullery Ct 61.;117-sx 3,364,087 1/1268 -'SO1OII1OI1 et al; 156--2 UX 3,574,012 4/1971Penberg 1s6 '11 UX 'EoREIGNPAT Nrs Y 923,134 4/19 3 G reatBritain-117'-s O'lHER REFERENCES New Scientist (No. 408), Etching Witha Laser,by

Wil1iam T. Reid, Sept. 10, 1964, pp. 648 and 64 9.

" WILLIAM A. POWELL, Primary Examiner v I U.S. c1. X.R. 101-1284; 156-7,253, 345; 161-112; 264--154

